Synthesis of ultrahydrophobic materials

Water and soil repellency has been one of the major targets for fiber and textile scientists and manufacturers for centuries. Combinations of new materials for fiber production with a variety of surface treatments have been developed to reach the condition of limited wettability. Nevertheless, additional efforts has been needed to create fiber and textile materials with ideal repelling properties. Nature has already developed an elegant approach that combines chemistry and physics to create super-repellent surfaces. Lotus leaves are unusually water repellent and keep themselves spotless, since countless miniature protrusions, coated with a water-repellent hydrophobic substance, cover their surface. Water cannot spread out on the leaves and it rolls around as droplets, removing grime and soil as it moves. 

The lotus effect is based on the surface roughness caused by different microstructures combined with hydrophobic properties of the wax covering the leaf surface. The surface roughness is the key prerequisite for the lotus effect. Owing to the rough surface the wettability of the lotus leaves is decreased and the contact area for dirt particles is reduced. 

Classical works of Wenzel and Cassie and Baxter established that roughness as well as surface energy are the factors that determine wettability. Wenzel proposed a model describing the contact angle 0 at a rough surface  

12 (a) Silver nanoparticles of size greater than 105nm adsorbed on PET fiber surface. Magnification x 10000. Static water contact angle on (b) control sample grafted with PS (no silver) and (c) ultrahydrophobic fabric. 

A typical static contact angle analysis was performed on polyester fabric modified with the silver/PS approach and on a control fabric modified with only PS (no silver). The contact angle of the fabric increased from 113° 4° (Fig. 17.12b) for control surface to 157° 3° for PS/silver multilayer system (Fig. 17.12c). The increase in contact angle was due to the partial contact of water with PS and entrapped air between silver nanoparticles. This synergistic effect of the hydrophobicity of PS and the roughness caused by silver nanoparticles indeed resulted in a contact angle beyond the superhydrophobic boundary. 

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